Position determination is required for the execution of a variety of activities. Some survey and mapping activities include determining the position of objects in the world, either near the position determination device or some distance away. Other activities include navigation or stakeout, where the coordinates of a destination is known, and the operator is guided to that destination by iteratively determining the position of a device and comparing that position with the destination coordinate. The position determination process in optical surveying typically starts from a known location, called a Point of Beginning (POB). The POB may have 2 or 3 coordinates (e.g., X, Y, and Z coordinates), in a specified coordinate system. Such a coordinate system may be a local one, where the location of a given starting point is given in “northing and easting” and height relative to a predetermined datum, or in latitude, longitude and height, or altitude. A typical optical survey system is set up and leveled directly over the POB, and the vertical distance between the POB and the instrument is measured, often with a tape measure. The system uses a visual sighting instrument in the form of a telescope with a built-in electronic distance measurement system to sight to a target located away from the POB. The telescope is coupled to a set of angle measurement devices that give elevation angle (also known as pitch) and a horizontal angle. The raw horizontal angle is relative, not absolute. To convert these relative horizontal angles into absolute azimuths (that is, with respect to true north), the first operation is to measure the horizontal angle in a known direction. This is usually accomplished by sighting to another known reference point, known as the backsight. These measurements are made on a tripod platform which enables the survey instrument to be manually leveled so that the vertical axis of rotation for horizontal angle determination is aligned with the local gravity vector. Leveling also ensures that the elevation angle is absolute (i.e., with respect to local gravity). Alignment of a local vertical axis on the measurement device with the gravity vector has been the standard way to create a reference plane from which azimuth and elevation angles can be measured. After measuring the angles and distance to a target, well-known geometric calculations determine the location of the first target relative to the POB. The survey process may continue by moving the survey instrument to directly over the first target location, repeating the leveling process for the tripod mounting system, and continuing to sight to the next target, again obtaining a range distance and angles to the new target.
The underlying assumption in the above data collection process is that the local vertical axis is well-behaved and well known, and serves as the common orientation reference by establishing a locally horizontal plane and vertical axis to which all range and horizontal angle measurements are referenced. The process always requires a re-leveling step for the survey instrument at each new target location, and the vertical distance between the new POB and the instrument is re-measured. There is quite an art to setting the instrument up so that it is both level and vertically above the POB. This setup procedure is time-consuming, and is a source of error if not accomplished accurately.
It should be noted that handheld laser rangefinders commonly measure absolute orientation and elevation angle, without leveling and backsighting. However this is achieved through electronic tilt sensors and magnetometers, which deliver only low accuracy measurements of roughly one degree, and so are not suitable for high accuracy positioning applications.
Total stations for surveying and construction measure relative angles to an accuracy between 0.5 and 10 arc-seconds; additional errors can be introduced by mis-levelling and imprecise target sighting as described above.
The introduction of Global Positioning System (GPS) and Global Navigation Satellite Systems (GNSS) receivers significantly reduced the need for optical surveying, as a 3D position can be determined from a single measurement of the GNSS receiver at a desired target location. But when high accuracy positioning is desired, as in most survey applications, the GNSS antenna still needs to be precisely positioned over the desired spot on the ground, or over some other physical target. Since GNSS operational practice dictates keeping the antenna at a known distance vertically above a ground target, often on a pole or rod about 2 meters in length, a different kind of leveling activity must be performed as well. Again, leveling a pole requires a bubble-level indicator as in the case of a survey tripod, and takes some time to bring the pole into a vertical alignment with sufficient accuracy to meet survey requirements. Further, there are many circumstances where the GNSS receiver cannot be located exactly at or over the target point.
GNSS receivers often do not give high accuracy positions in environments where the satellites are not clearly in view, such as near buildings or trees. In such cases an optical instrument such as a total station can be used. Alternatively a GNSS receiver can be combined with a total station, where the GNSS position is used to avoid the need to find an existing local POB. However the operator of a combined GNSS total station still needs to level it and take a backsight reading.